An electric double layer capacitor is an electrical storage element in which the capacitance of electric double layers formed by adsorbing/desorbing electrolyte ions on interfaces between a pair of positive and negative polarizable electrodes and an electrolyte solution is utilized.
Small capacity type capacitors for memory backup etc., intermediate capacity type capacitors and large capacity type capacitors such as auxiliary power supplies for electric vehicles, auxiliary power supplies for solar batteries, auxiliary power supplies for wind power generation, and momentary interruption compensation devices have been widely examined for applications of electric double layer capacitors.
An electric double layer capacitor has characteristics such as a capacitance (C), a working voltage (V), an energy density (E), an internal resistance and a working life. Improvement in all the characteristics is important; however, recently, improvement in energy density which is the disadvantage of electric double layer capacitors and further prolongation of a working life (improvement in long-term reliability) which is their advantage have also been particularly demanded in the applications of the above-mentioned small to large capacity types.
Since the energy density (E) of an electric double layer capacitor is proportional to the square of a working voltage (energy density E=0.5×C×V×V (wherein C: capacitance, V: working voltage)), increase in working (withstand) voltage is effective for improvement in energy density.
Electric double layer capacitors are those in which electrolyte solutions are based on water and not based on water. When the water-based electrolyte solution is used, there is an advantage that a low-resistance-type electric double layer capacitor is obtained, but it is necessary to control a working voltage to not more than a voltage at which electrolysis of water occurs, and it is used at around 0.8 V. In contrast, the non-water-based electrolyte solution can be used at a higher voltage, and it is used usually at a working voltage of around 2.5 V, and at a higher voltage of around 2.7 V. Recently, a capacitor used at a higher working voltage and having a withstand voltage of 3.0 V or more has been demanded. This is because, when a plurality of capacitors are connected and used in the application of a high voltage, increase in the withstand voltages of the capacitors results in reduction in the total number of cells used, reduction in cost, and savings in module space. However, there has been a risk that increase in working voltage might accelerate the time degradation of an electric double layer capacitor (decrease in capacitance over time and increase in resistance over time) and a problem of deteriorating long-term reliability which is a characteristic of the electric double layer capacitor.
There are various reasons for the time degradation of an electric double layer capacitor, which are considered to be the following mechanisms. In the electric double layer capacitor, residual moisture in an electrolyte solution and moisture mixed during assembling are present. Also, physical adsorption water and chemically bonded adsorption water exist, on the surface of, and in an activated carbon, a conductive assistant, a binder, a separator and the like, which are used in an electrode layer. These kinds of moisture are desorbed as water during repeating discharge and charge and are electrolyzed by applying a voltage to generate gases, to result in decomposition of an electrolyte (solution), and to generate decomposition products. These decomposition products reduce the surface area of the activated carbon to result in reduction in capacitance by coating the surface of the activated carbon and clogging the pores of the activated carbon. Also, these decomposition products result in increase in resistance by clogging the opening of the separator.
Therefore, reduction in moisture, suppression of reduction in the surface area of an activated carbon by a decomposition product, and suppression of decomposition of an electrolyte solution have been considered to be effective for developing a high-withstand-voltage electric double layer capacitor. These development techniques are considered not to be independent of each other but to be related to each other. Further, since there are a plurality of mechanisms of time degradation of an electric double layer capacitor as mentioned above, only the reduction in moisture, the suppression of reduction in the surface area of an activated carbon by a decomposition product, and the suppression of the decomposition of an electrolyte solution are not techniques effective for developing a high-withstand-voltage electric double layer capacitor.
The avoidance of the decomposition of an electrolyte solution has been examined but has been considered to be difficult, because activated carbon used in an electric double layer capacitor has a catalytic action because of having a large surface area for increasing an adsorption capacity. It is said that the catalytic action is caused by a functional group on the surface of the activated carbon (see Non Patent Literature 1).
As a method for reducing moisture in an electric double layer capacitor, the higher withstand voltage and longer life of the electric double layer capacitor is reported to be achieved by, for example, improving the material of a separator to develop the separator that is not deteriorated even on a high temperature and long-term drying condition (for example, see Patent Literatures 1 and 2). The drying temperature condition can be enhanced by improving the separator. However, there have been problems that an organic matter component used in another configuration site in the electric double layer capacitor, such as a binder or an adhesive layer, is deteriorated to coat an activated carbon or the separator or clog pores to result in decrease in capacitance and that a resistance is increased with separation of an interface between an electrode and a collector. Further, heat treatment in a vacuum or under an inert gas air stream at several hundred degrees Celsius is necessary for completely removing moisture in the pores of the activated carbon and a functional group on the surface of the activated carbon, and this method has not been practical in light of a cost.
In addition to this, a high-withstand-voltage electric double layer capacitor excellent in long-term reliability is reported to be obtained by subjecting a carbon fiber woven fabric to activation treatment and thereafter to heat treatment at 700 to 1000° C. under inert gas atmosphere to remove moisture adsorbed in the activated carbon fiber woven fabric and a surface functional group (for example, see Patent Literature 3). The moisture adsorbed in the activated carbon fiber woven fabric and the surface functional group can be removed by subjecting the activated carbon fiber woven fabric to the heat treatment at 700 to 1000° C. under the inert gas atmosphere. However, there is a problem that the readsorption of moisture and the reintroduction of a surface functional group occur unless the activated carbon fiber woven fabric after the removal is stored under a completely dehumidified situation to assemble the electric double layer capacitor, and this method is not practical.
In addition to this, there is reported an example in which the decomposition of an electrolyte solution is considered to be caused by an acid due to moisture generated in a positive electrode and various antacids are added into an activated carbon polarizable electrode (for example, see Patent Literature 4). Further, there is reported an example in which the decomposition of an electrolyte solution is considered to occur due to hydrogen ions generated by electrolyzing generated moisture and hydrogen ions are reduced by reacting with or absorbing hydrogen ions, for example, an aluminum powder is added into an electrode (for example, see Patent Literature 5). However, there is a problem that an antacid sometimes decomposes and reacts with an electrolytic solution during the discharge and charge of an electric double layer capacitor. In the case of the addition of the aluminum powder, aluminum may react with electrolyte ions to synthesize, e.g., aluminum fluoride or the like. Aluminum fluoride, which is an insulator, may increase a resistance and is not too preferable.
The generation of a gas due to a surface functional group and reaction with an electrolyte solution are reported to be suppressed by reacting the surface functional group of a carbon material, which is a main component of a polarizable electrode, with an organosilicon compound to cap it with a chemical bond (for example, see Patent Literature 6). The method of capping the surface functional group of the activated carbon by using the organosilicon compound has had a problem that not only the surface functional group is capped but also the pores of the carbon material are clogged depending on production conditions and, as a result, a capacitance is reduced.
The present inventors have proposed that an electrode for a high-withstand-voltage-type electric double layer capacitor with a high energy density and less degradation over time in capacitance and resistance, that is, excellent long-term reliability, and the electric double layer capacitor could be provided by adding tungsten oxide to an electrode material (see Patent Literature 7).